1. Field of the Invention
This invention relates to flow meters and particularly to a temperature-sensitive flow-meter capable of measuring and digitally counting the instantaneous flow rate and cumulative flow rate of fluid.
2. Prior Art
In modern industrial technology, pipe lines are extensively used to convey a wide variety of fluids. When these conveyance systems are computer controlled, the need arises for accurate digital measurements of the fluid flow rates. Such data are used not only to monitor flow, but as feedback information for the digital computer controlling the flow.
The use of a thermal flow-meter is an effective approach to determine fluid flow rates. These devices employ thermally responsive transducers which provide analog signals in accordance with the rate of flow. Known prior art thermal flow-meters directly produce only analog non-linear varying output signals. Illustrative of such an approach is the flow-meter disclosed in U.S. Pat. No. 3,372,590. As is therein disclosed, a transducer comprising a thermistor is disposed in the path of fluid flowing through a conduit. Constant power is applied to the thermistor maintaining it at a fixed temperature above that of the fluid. The fluid flowing past the thermistor will tend to cool it and thereby induce an impedance change.
Other prior art known at the present time consist of U.S. Pat. Nos. 3,220,255 and 2,947,938; an article entitled "Mass Flow Measurement" by Robert Siev in Instrument & Control Systems, June 1960, page 966; and an article entitled "The Hot Wire Anemoneter" by O. C. Wolf in the same journal on page 960.
Generally, such thermal flow-meters will measure mass flow rates with speeds and accuracies sufficient for some applications, such as coarse monitoring of simple conveyance systems. However, none of the known prior art devices has produced directly a truly satisfactory digital output.
The apparatus of the present invention produces a digital output proportional to fluid mass flow rate. The advantages of direct digital measurement are numerous. The obvious advantage is that the digital information can be applied directly to and be processed by computers. An additional advantage is that no moving parts are utilized in the flow-meter. In many industrial applications the lack of moving parts can be very critical. A further advantage is that the design of the flow-meter provides inherent protection of the sensing elements against thermal damage. Another advantage is that the signal readout is not subject to a "reading error" as in analog instruments. Still another advantage is the insensitivity to noise.
The present invention can achieve overall accuracies in the order of 0.5% while providing a linear response over a broad range of fluid flow. Moreover, both a linear relation between the flow rate and the electrical power to the transducers, and a uniform sensitivity for flow rates measured over a broad range can be achieved.